Adsorptive denitrogenation of model fuels with porous metal-organic frameworks (MOFs): Effect of acidity and basicity of MOFs

To understand the effect of the acidity or basicity of porous metal-organic frameworks (MOFs) on the adsorptive removal of nitrogen-containing compounds (NCCs), an MOF (MIL-100(Cr)) was modified to impart acidity or basicity onto the MOFs. The modification was done by grafting ethylenediamine and aminomethanesulfonic acid onto coordinatively unsaturated sites of the MOF, MIL-100(Cr). The adsorptive removal of a basic quinoline or benzothiophene can be improved noticeably, especially at low concentrations, with the introduction of an acidic site; however, a basic MOF causes a severe decrease in the adsorptive performance for a basic adsorbate such as quinoline. The effect of the interaction of the base-base on adsorption was more severe or detrimental for a hard base quinoline than for a soft base benzothiophene. Functionalized MOFs show a slightly decreased adsorption for a neutral adsorbate such as indole probably because of the decreased porosity of the MOFs compared with the virgin MOF without functionalization. Moreover, a functionalized MOF (with SO₃H group) can be used several times after simple washing with acetone. From the present research, it may be concluded that acid-base interactions between NCCs and MOFs will lead to favorable adsorptive removal of NCCs. However, for the adsorptive removal of a neutral adsorbate such as indole, another adsorption mechanism (such as π-complexation or hydrogen-bonding) is needed for high uptake and efficient removal.